Modeling Study to Evaluate the Ionic Mechanism of Soot Formation
Annual rept. 15 Dec 1990-14 Dec 1991
AEROCHEM RESEARCH LABS INC PRINCETON NJ
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It has been demonstrated that in a sooting laboratory flame the time to add a carbon atom to a growing carbon species is about the same, or shorter, for the ionic than for the free radical mechanism. The calculated step time is consistent with the experimentally observed time in the flame to build a soot particle. It has also been demonstrated that, up to about 30 carbon atoms, the thermodynamic driving force for the ionic mechanism is much greater than for the neutral mechanism above 30 carbon atoms they are equal. The Langevin theory of ion-molecule reactions has been modified to accommodate large ions. The results indicate a greater collision coefficient for large ions than calculated by Langevin. The coefficient for a hard sphere collision decreases as ion diameter increases up to about 3 nm diam for a conducting spherical ion, and about 1 nm for a nonconducting spherical ion. The Sandia Flame Code has been modified to 1 accept individual experimental concentration profiles as input 2 use non-Arrhenius rate constants and 3 accept ambipolar diffusion coefficients. Analysis of our previous modeling of the ionic mechanism and many runs with the modified code indicate either a basic flaw in the mechanism or a modeling problem. The detailed modeling results are inconsistent with the calculated speed with which carbon atoms are added and with the strong thermodynamic driving force for the ionic mechanism.
- Atomic and Molecular Physics and Spectroscopy
- Combustion and Ignition